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The underdrive pulley was developed for motorsport applications to preserve the power steering pump and slow the accessories on a motor that spends a lot of it's time at high revs (4-6K). A side benefit is that it reduces parasitic drag so you get a bit more torque/hp to the rear wheels at all rpm.

If you spend long hours stuck idling in traffic with headlights on and the stereo cranked you probably don't want it. If your car spends most of it's time between 2-3k rpm, you don't want it. If your car is a weekend joyride/track toy and you use all of your rpm range, this is right up your alley.